GB2098368A - AC to DC converter circuit - Google Patents

Abstract

The converter circuit converts AC input voltage within the range of 85- 260 VAC to a constant voltage, current limited DC output voltage, all without the use of a transformer or input voltage-select switch. The AC input voltage (at L1, L2) is supplied directly to a rectifier (1) and then to the input of a phase-controlling element (SCR) for controlling the phase of the unfiltered rectified signal. A trigger signal-generating circuit (3) generates a trigger signal for regulating the operation of the phase- controlling element. A ripple-removing filter circuit (4) receives the output from the phase-controlling element and removes the ripple component. A voltage-regulating circuit, a current- limiting circuit, and a rapid start-up circuit to facilitate rapid generation of DC output signal soon after energization of the overall circuit by an AC power source are also disclosed. <IMAGE>

Description

SPECIFICATION AC to DC convertor circuit The present invention relates to a power supply circuit for converting AC to DC, and more particularly to a circuit for obtaining a low voltage, stable DC signal from a non-switched successive and variable alternating current electrical source, having a broad range from 85 to 260V, without use of a step-down transformer or tap-switch, so that circuits of electronic appliances can be protected.

As is commonly known, the available AC input voltage varies from area to area. Presently, the usual AC electrical source used in Korea and Japan is 1 OOV and/or 220V. In some countries, including Australia, 250V AC power is used, and AC power from 100V--150V is usually used worldwide. (Table I below lists by country the type of available AC power.) Therefore, when electronic appliances are exported to other countries where the AC electrical source has a different voltage, a power supply using a transformer having a fixed voltage specification, depending on the area where it is going to be exported, is usually necessary.

This variation in AC power from country to country causes inconvenience in making electronic appliances and thereby results in both increase of production costs and other additional problems.

A conventional power supply is illustrated in Fig. 1, wherein a tap-switch SW2 is switched to one of two positions as determined by the input voltage of the available alternating current (for instance, 1 OOV, 220V) which is to be inputted into input terminals 1 a and 1 b. At the output side of the step-down transformer T, a predetermined AC output voltage is obtained when the switch SW1 is closed. This output voltage is rectified by a resistor Ra and a rectifier bridge Rf and then is partially smoothed by a capacitor Ca. This partially smoothed DC voltage is further filtered by an active filter circuit comprising transistors Qa, Qb and Qc, for the purpose of removing ripple caused by the frequency of AC voltage.

However, in the conventional circuit mentioned above, a step-down transformer and a tap-switch (for setting of either 1 00V or 220V) is necessary.

The conventional circuit is usually included inside an electrical appliance (for example, TV, audio set, video set, etc.) in case of a variation of AC input voltage depending on the area where the appliance is used. Frequently, in order to set the tap-switch, the appliance cabinet must be disassembled. If it is not properly set, troubles in electronic appliances may occur. Also, this design may cause difficulties in handling, as well as increasing the cost of production and the weight of the appliance. Further, because some electrical power is lost in the transformer T, its efficiency is very low. Efficiency of DC output compared to AC input is about 70 percent and the remaining 30 percent appears as a loss of electrical power in the circuit.

It is an object of the present invention to provide a converter circuit which obviates or mitigates these difficulties.

The present invention is an AC to DC converter circuit for use with AC supply signals of various voltages, comprising a rectifying circuit which produces rectified current at its output when connected directly to a source of alternating current having any voltage within a broad range; a phase-controlling element connected to the output of the rectifying circuit for controlling the phase of the rectified current signal; a trigger signal-generating circuit for generating a trigger signal for regulating the operation of the phasecontrolling element, and a ripple removing filter circuit connected to the output of said phasecontrolling element for removing the ripple component of the DC signal and for providing a relatively constant and substantially ripple-free output signal at an output terminal for energizing an electrical appliance, whereby the amplitude of the DC output signal is independent of the amplitude of a directly connected AC input source.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is an electrical schematic of a circuit for an electrical source using a conventional stepdown transformer and voltage select switch; Fig. 2 is a block and schematic diagram showing an embodiment of the present invention; Fig. 3 is an electrical schematic showing in more detail the Fig. 2 embodiment of the invention; Fig. 4 is an electrical schematic of another embodiment of the present invention; Fig. 5 is an electrical schematic of an active filter circuit according the present invention; Fig. 6 is an electrical schematic of an active filter circuit according to the present invention; and Fig. 7 is a diagram of certain voltage waveforms at certain points of the circuit of Fig. 3.

The present invention will now be explained in detail with reference to the accompanying drawings.

Fig. 2 shows a schematic diagram of an embodiment according to the present invention.

Input terminals L1 and L2 receive an AC voltage supply and pass this AC signal through SW1 (when closed) and the fuse to rectifying bridge circuit 1. The anode of rectifying circuit 1 is connected to an SCR (for controlling the phase) through R1 and choke coil 2 which extends the operating time of the SCR. The resistor R2 is connected to the trigger signal generating circuit 3 which triggers the SCR for controlling the phase in order to control each cycle of the AC input signal. The cathode terminal of the SCR is connected with the capacitor C1. The SCR charging signal is fed back to the trigger signal generating circuit 3 according to the charge of the capacitor C1, and is also connected to the input of the ripple-removing filter circuit 4. The output signal of the direct current shows substantially no ripple at point E (Fig. 7).

Fig. 7A shows the waveform at point A which is the output from bridge rectifying circuit 1. The trigger signal generating circuit 3, shown in greater detail in Fig. 3, receives this pulsating wave, which has a constant low level voltage because of resistor R2, capacitor C2, and reference voltage (Zener) diode D1.

The pulse voltage at point A is divided by a voltage divider comprising resistors R3 and R4, and then is supplied to transistor Ol through capacitor C3 and diode D10. At the collector of transistor Ol (point D) appears substantially the same wave voltage as that shown in Fig. 7A, because of the time constant of resistor R5 and capacitor C3. This voltage wave is transmitted to the base of transistor Q2 through resistor R8 and an integrating circuit, consisting of resistors R6, R7 and capacitor C4, allows transistor Q2 to conduct. When conducting, the potential of the transistor Q2 at its emitter is about 0.7V higher than the potential of its base. Resistor R9 is connected to the emitter of transistor 02, whereas resistor R8 is connected to the base.

The potential of the emitter of transistor 02 is controlled by energizing transistor Q4 through resistor R10. Transistor Q4 is activated upon a detection of error by the potential of the base through the feedback circuit, when a DC voltage signal containing ripple appears at capacitor C1 connected to the cathode terminal of the SCR.For example, when the DC voltage at point C, (as shown in Fig. 7C) through capacitor C1 is higher than a predetermined value, the base potential of transistor Q4 is energized so that electrical current flows through the collector and through resistor R 10. At this time, the potential of the emitter of transistor 02 is lowered, transistors Q2 and Q3 no longer conduct and a voltage potential change occurs across resistor R14. This voltage is differentiated by capacitor C5 and diode D2 and appears as a negative going pulse type voltage signal which is amplified by transistor OS after passing through resistor R15. The pulse current amplified by transistor Q5 generates a synchronized trigger signal, as shown in Fig. 7B, which is transmitted through resistor R16 and capacitor C6 to the gate of SCR for turning the SCR off. The capacitor C1 is then charged and the pulse voltage, as shown in Fig. 7A, is decreased to zero to turn the SCR off The potential of the SCR anode terminal is decreased to zero. The choke coil 2 functions to extend energizing time of SCR and prevents unwanted radiation from being generated when the switch is operated.

When potential at point C is decreased to less than a predetermined value, the base voltage of transistor 04 is decreased to less than the bias voltage, and then transistor Q4 is turned off. The potential at the emitter of transistor 02, through resistor R9, is increased to energize transistor Q2, and the bias potential is transmitted to the base of transistor Q3 through resistor RI 7. A pulse current is generated at the collector of transistor OS to trigger the SCR. The SCR is energized to charge the capacitor C1 and the stored charge in the capacitor C1 is increased.

As described above, when the stored charge in the capacitor C1 is more than a predetermined value, the transistor Q4 is activated through a feedback circuit consisting of resistors Rl 1, R12 and R13 to turn off the transistor Q2, and thereby turn off the SCR by operating the SCR trigger signal through transistor OS to end the charging of the capacitor C1. When the stored charge of the capacitor C1 is decreased to less than a predetermined value, the transistor 04 is turned off, the transistors Q2 and Q3 are rendered conductive, and the SCR is turned on by generating a trigger signal with transistor Q5, to thereby charge capacitor C1.This operation is repeated for each cycle, so that the wave voltage as shown in Fig. 7C appears at point C across the capacitor C1.

Since the voltage across the capacitor C1 contains ripple, a ripple-removing filter circuit 4 for the purpose of removing the ripple from the signal is provided, consisting of an active ripple filter circuit and an over-current limiting circuit.

Fig. 3 shows one of the examples of a rippleremoving filter circuit 4 which filters the signal as shown in Fig. 7C to obtain a direct current signal (Fig. 7E) which contains less than a 0.05 percent ripple component across capacitor C8.

After the voltage across capacitor C1 decreases to a certain level (after a certain time period as determined by a time constant dependent on the multiplied values of resistor Rl 7a and capacitor C7), it is transmitted through resistor R18 to activate transistors 06, 07 and Q8 (arranged in a Darlington configuration) and is maintained at a constant voltage level by reference voltage (Zener) diode D3. Resistor R19 is connected to the emitter of transistor Q9 and resistor R20 is connected to the base of transistor Q9 to act as bias resistor for transistor 09. When the current at the emitter of transistor Q6 through resistor R20 is more than a predetermined value, a voltage transmitted to the base of transistor Q9 through resistor R20 turns on transistor 09.Thus, the base current of transistor Q6 is shunted to limit current and the output voltage corresponds to the charge voltage of the capacitor C1, so that a constant level DC voltage of direct current with a constant level as shown in Fig. 7E can be obtained at the positive terminal of capacitor C8.

Resistors R21, R22, R23 and R24 are bridge resistors and diode D4 serves to protect transistors 06, Q7 and 08.

The ripple-removing filter circuit 4 as shown in Fig. 3 requires a certain time (as determined by multiplied value of resistor Rl 7a with capacitor C7) from the time that an electrical source is inputted to the time of a predetermined normal activation of the ripple-removing circuit. It has been experienced that electronic appliances are difficult to activate rapidly because of the relatively long time constant of the resistor Rl 7a and capacitor C7, whose values are chosen in order to obtain an output signal at capacitor C8 (i.e., output as shown in Fig. 7E), which has minimum ripple.

Accordingly, Fig. 5 shows a second example of a ripple-removing filter circuit 41 which, compared to the ripple-removing filter circuit 4 of Fig. 3, is improved so that electrical appliances can be activated and energized in a shorter time.

The ripple-removing filter circuit 41 of Figure 5 comprises the same resistors (R17a, R18, R19, R20, R2 1, R22 and R23), transistors 06, 07, Q8 and Q9), capacitors (C7 and C8), and diodes (D3 and D4) as in the first example shown in Figure 3.

In addition, the circuit of Figure 5 also comprises diode D5 and resistor Ri 8 to the base of transistor Q8. Also, diode D4 is connected between the diode D3 and the resistor R19, and resistors R26 and R27, along with the capacitor C9, are connected to the anode side of diode D6.

The output from the ripple-removing filter circuit 4 of Figure 3 is retarded, relative to the input, by a time period which is related to the tim constant oi the circuit comprising resistor R 7a and capacitor C7. However, in circuit 41 of Figure 5, the anode potential of diode D6 is established to reach a normal operating value in a shorter time than the anode potential of diode D5. In an initial overcurrent condition, voltage through resistor R26, capacitor C9 and diode D6 is transmitted to the base of transistor 08.After the lapse of a certain time period (as determined by time constant of circuit comprising resistor 1 7 and capacitor C7), the anode potential of diode D5 has a slightly higher voltage value than the anode potential of diode D6 so that diode D6 is turned off, not to be activated, and voltage through diode D5 is transmitted to the base of transistor Q8 to conduct transistors 06, 07 and 08. Therefore, by changing the time constant of the circuit an electronic appliance connected to the output at point E can be more rapidly activated by greatly shortening the start-up period (as compared to the above arrangements of Figure 3 with activation solely through resistors R17 and R25, capacitor C7 and diode D5), and also in an advantageous manner to minimize ripple content.

Figure 6 shows a third example of rippleremoving circuit which comprises a filter circuit 42 having an over-current limiting circuit. With the circuit of Figure 6, the ripple-removing filter circuit 42 is increased to 220 win overvoltage just after the initial input of an electrical source when a phase controlling SCR or its trigger signal generating circuit 3 is not normaily activated. The capacitor C1 is charged up to the peak value in response to an inputted electrical source of alternating current, for example, when it is activated with 220V of alternating current, but the DC output at point E is established at a relatively low voltage value relative to the high voltage between the collector and emitter of transistor Q6.The allowable value of current in the collector is preferably limited, and at this time a high value of voltage to charge capacitor C1 can result in problems because it takes a long time to discharge the capacitor through the high resistance which includes resistors R17a and R18.

In order to shorten the capacitor discharge time, a higher value of limiting current in the emitter in transistor 06 is obtained. However, a relatively precise and appropriate value of limiting current is preferably established in view of stable activation of transistor 06. In this example (of Figure 6) resistors R17, R18, R19, R20, R21, R22, R23, R24, transistors Q6, Q7, 08, 09, capacitors C7, C8, and diode D3 are similar to those described in the first and second examples.

Figure 6 shows a third example of a ripple removing filter circuit 42 in which resistor Ri 9 of the second example is replaced by resistors R29, R30 and R31,which are connected in series between the emitter of transistor 06 and capacitor C8. The voltage at the junction of resistors R29 and R30 is inputted to the base of transistor Q9 through diode D7 (and resistor R31 in parallel therewith), resistor R33 and diode D8.

Resistor R28, through reference (Zener) diode D9, is connected to the anode of diode D8.

This arrangement of the ripple-removing filter circuit 42 limits initial overcurrent. In this arrangement, a current-limiting circuit consisting of resistor R28, R29 and R32, and transistor Q9 is activated so as to limit the current under operating conditions which satisfy the following equation. The voltage values of course depend upon the input voltage.

E1 +E2=0.6V (V of transistor Q9) (1) where E, represents the voltage drop across resistor R29 and where E2 represents the voltage drop across resistor R32.

Transistor Q9 is preferably a silicon transistor.

Where the potential of point C and point E is at its normal operation level, current limiting starts under the following conditions: E=0.6V E2=OV.

If the potential of point C has markedly increased, current limiting starts under the following conditions: E,=OV E2=0.6V.

The voltage drop across resistor R29, i.e. E1, is generated by charged value of electrical current.

When E, is about OV, it is very difficult to obtain a precise value of limiting current, because a transistor 06 having stable operation over a large range is necessary. Since the potential at point C contains much ripple and affects the direct current output through resistor R28, the resistors R30 and R31 are connected in series with resistor R29, and limit current under conditions satisfying the following equation (2), whatever the conditions of input voltage are.

E1+ E2+E3+E4=l .2V=(V of transistor Q9+ V of diode D8) (2) Wherein E, represents the voltage drop across resistor R31, E2 represents the voltage drop across resistor R30, E3 represents the voltage across diode D8, E4 represents the voltage drop across resistor R33, and E5 represents the voltage drop across resistor R29. If the potential at point C and point E are at their normal level of operation, current limiting starts under the following conditions according to equation (2): E1=0.6V, E2=0. 6V, E3=OV, E4=OV.

If the potential at point C is markedly increased, each resistance value is established to start current limiting under the following conditions: El=OV, E2=OV, E3=0. 6V, E4=0.3V, wherein E, and E2 in equation (2) represent voltage generated mainly by charging current, and E3 and E4 represent the potential which is divided across resistor R28 and diode D9.

Furthermore, the value of resistor R30 is chosen to be several times that of resistor R3 1.

Diode D10, which is a silicon diode, has a typical forward voltage Vf of about 0.9V, which voltage is divided by resistors R29 and R30 at a dividing ratio chosen to be about 1:2.

Diodes D7 and D8 are silicon diodes and have forward voltage Vf of about 0.6V. Diode D10 can have a reference voltage in the range of about 1 0-50V (an even greater range may be preferred), and prevents transmission of ripple voltage through resistor R28 to the output at point E.

Accordingly, the value of current of a limited charge depends mainly on a lowered voltage value of resistor R30 when an input voltage overcurrent is present at point C, i.e. upon input of an extremely high initial current.

Under such conditions, the current value of limited charge is able to be maintained at a relatively stable and precise value, which is satisfactorily maintained even under extreme values of input voltage. When the voltage at point C on the input side of the ripple-removing filter circuit 4 is extremely increased, it is operated at a normal level by discharging in a short time.

Therefore, a rapid start of electronic appliances can be effected and the output is not affected by ripple voltage at the front end.

Figure 4 shows an alternative SCR arrangement designed to rectify an alternating current, wherein two SCRs are employed, and are placed before the resistor R1 and coil 2. The other parts of the circuit of Figure 4 are explained above.

As described above, the present invention provides a virtually perfect signal of direct current, where a given stable ripple content is no more than about 0.05 percent. This DC output is obtained by completely removing virtually all ripple through a ripple-removing filter circuit. This is accomplished without a step-down transformer or select switch on the input terminal of alternating input current. Also, the present invention has the advantage that a given constant voltage of DC electrical source is steadily and successfully obtained under AC input voltages ranging from 85V-260V, all without the need for adjusting and switching devices.

As shown in Table 1 as a reference, even in areas where the electric source has different voltage values, the circuit of this invention is very effective. Because parts such as step-down transformers are not used, the loss of electrical power in the circuit itself is low and the output power of the AC input, resulting in a relatively high efficiency.

Table 1 usual voltage Frequency Country (V) (Hzl Algeria 127/220 50 Australia 240/250 50 Bahrain 230 50 Bangladesh 220/230 50 Bolivia 110/115/220/230 50/60 Brazil 110/121/220 50/60 Taiwan 220 50 Colombia 110/115/120/130 60 France 115/127/220/230 50 Guatemala 110/120/127/220 60 Hawaii 115 60 Hong Kong 200 50 Italy 127/1 60/220 50 Japan 100 50/60 Kuwait 240 50 Mexico 110/120/125/127 50/60 U.S.A. 117 60

Claims (7)

Claims

1. An AC to DC converter circuit for use with AC supply signals of various voltages, comprising a rectifying circuit which produces rectified current at its output when connected directly to a source of alternating current having any voltage within a broad range; a phase-controlling element connected to the output of the rectifying circuit for controlling the phase of the rectified current signal; a trigger signal-generating circuit for generating a trigger signal for regulating the operation of the phase-controlling element, and a ripple-removing filter circuit connected to the output of said phase-controlling element for removing the ripple component of the DC signal and for providing a relatively constant and substantially ripple-free output signal at an output terminal for energizing an electrical appliance, whereby the amplitude of the DC output signal is independent of the amplitude of a directly connected AC input source.

2. A circuit as claimed in claim 1, wherein the phase controlling device is an SCR.

3. A circuit as claimed in claim 1 or claim 2, further including a choke coil for extending the energization time of the phase controlling device.

4. A circuit as claimed in any preceding claim, wherein the ripple-removing filter circuit comprises a resistor and capacitor connected in series between the input of the ripple-removing circuit and ground, and at least two transistors arranged in Darlington configuration having their collectors connected to the input of the rippleremoving circuit, the base input of the first Darlington transistor connected to the junction of the resistor and capacitor, and the emitter output of the last Darlington transistor connected to the output of the filter circuit, said filter circuit also having regulating means comprising a regulating transistor having its collector connected to the junction of the resistor and capacitor and to the emitter of the last Darlington transistor configuration through a diode, said regulating transistor conducting to shunt current when the potential at the emitter of the Darlington configuration is above a predetermined value.

5. A circuit as claimed in claim 1, wherein the ripple-removing filter circuit comprises a first resistor and first capacitor connected in series between the input of the ripple-removing circuit and ground, at least two transistors arranged in Darlington configuration having their collectors connected to the input of the ripple-removing circuit, the base input of the first Darlington transistor connected to the junction point of the first resistor and first capacitor through a first diode, and the emitter output of the last Darlington transistor connected to the output of the filter circuit, said filter circuit also having rapid startup means for conducting the Darlington configuration transistors more quickly relative to the time constant of the first resistor and first capacitor, said means comprising a second resistor and second capacitor connected in parallel with, and having a time constant shorter than, the first resistor and first capacitor, and a second diode connected between the base input of the first Darlington transistor and the junction point of the second resistor and second capacitor, said second diode having a smaller forward voltage value than said first diode.

6. A circuit as claimed in claim 4, wherein the ripple-removing filter circuit further includes current limiting means to limit output current comprising a pair of voltage dividing resistors connected in series between the emitter of the last Darlington transistor emitter output configuration and the filter circuit output, the junction of the voltage dividing resistors being connected to the base of the regulating transistor through a parallel combination of a resistor and diode, a series resistor and series diode, with the junction of the series resistor and series diode connected to the input of the ripple-removing filter circuit through a Zener reference diode and resistor, whereby the regulating transistor conducts to limit current during an initial start-up condition.

7. An AC to DC converter circuit substantially as hereinbefore described with reference to, and as shown in, Figs. 2 to 7 of the accompanying drawings.